274 



THE DOCTRINE OF DESCENT. 



we encounter the remains of two Ungulate families, the 

 Palaeotheridse and the Anoplotheridse, essentially distin- 

 guished from one another by their dentition, and form- 

 ing the starting-points of the groups of Ungulates of 

 which some now appear so greatly isolated. The root to 

 which these two families lead back is unknown; on the 

 other hand, partly from the direct comparison of these 

 genera with the present Ungulata, partly from numerous 

 intermediate links found in the Miocene, Pliocene, and 

 Diluvium, it appears that, in the lapse of time, the separa- 

 tion which characterizes the present age was initiated, 

 and the seeming isolation was produced by the extinc- 

 tion of the intermediate links. It was this isolation which 

 induced the older systematizers to institute three orders 

 of Ungulata. 



The special pedigree emanating fromthePalaeotheridae 

 includes, among the present Ungulata, the horse, tapir, 



i^and rhinoceros. The transi- 

 / tion from the Palseotherium 

 ' to the horse may be directly 

 traced, and this, moreover, 

 in the two most important 

 M characters, the dentition and 

 the feet. In the Anchithe- 

 rium and Hipparion, the 

 transformation from the tri- 

 dactyle to the unidactyle 

 Ungulate is accomplished ; 

 and Riitimeyer's brilliant re- 

 searches have shown how, in 

 the milk dentition of each 

 genus, the definitive dentition of the aboriginal genus 



FIG. 26.— Skeleton of the foot. (P) 

 Anchitherium. (H) Hipparion. 

 (E) Horse. 



